Nail It Up, Call It a Problem
Eric, one of the owners of Hansen Pole Buildings, was contacted by a prefabricated roof truss manufacturer recently. They weren’t interested in selling just trusses; they wanted to provide their nail-laminated posts as well.
Among their selling points were the columns were, “stronger than even the glu-laminated columns”. They Emailed to Eric some nice glossy literature, which Eric forwarded to me, knowing how I love to pick things apart.
The first thing I noticed is, their literature is using values for the #2 Southern Yellow Pine (SYP) 2x6s, which their columns are made from, based upon the 1997 NDS (National Design Specification). However, the strength values for dimensional SYP have been downgraded as of June 1, 2013. This means the values listed in their literature are 25% too high in bending and about 15% in compression!
As manufactured by this company, the #2 SYP is used for the bottom pressure treated portion of the column, and the upper is #2 SPF (Spruce-Pine-Fir). The SPF has an Fb (fiberstress in bending) value of 1138 psi (pounds per square inch) compared to an Fb of 1000 for the 2×6 SYP.
Interestingly, the company DOES only take credit for the section modulus (Sm) in the spliced are as being 2/3 of what three 2×6 unspliced would provide. AS we will soon prove, this 2/3 is being pretty generous to them.
Dr. Frank Woeste’s “Nail Laminated Wall Columns from Dimensional Lumber” (see TRANSACTIONS of the ASAE Volume 27, Number 4, pp. 1127-1130, 1984) compared the strength of nail-laminated posts, with internal non-reinforced butt end splices.
In Woeste’s testing three-ply 2×6 #2 Dense SYP nail laminated posts, were compared in strength to 6×6 #2 SYP solid sawn columns. The moment resisting ability of a wood member is calculated from Fb X CD (duration of load = 1.6 for wind) X CM ( = 0.85 wet service factor for dimensional lumber) X Cr (repetitive member factor for 3 members joined together) X Sm. As the testing was done on lumber which had not been subjected to moisture CM will be disregarded for comparisons.
For 2×6 #2 Dense SYP (based upon 1985 values) 1450 X 1.6 X 1.15 X 3 members X 7.5725 = 60,530 in-lbs. For 6×6 #2 SYP 850 X 1.6 X 27.73 = 37,712 in-lbs. Therefore, the 3 2×6 (no splices) would be 60% stronger than the solid sawn 6×6.
In the study, using the butt spliced columns, the 3 2×6 #2 Dense turned out to be only 64% as strong as a 6×6! Whoops…..
Using the results of this scientific study, it would appear non-reinforced butt spliced columns should probably be used at a value of somewhere around 40% of the strength of non-spliced columns. While I know there are some folks (those who use or produce the non-reinforced butt spliced columns) who are not going to like to read this – the facts are indisputable.